In a conventional three-dimensional object building apparatus (SLM (Selective Laser Melting) apparatus), which is known for example as shown in Patent Document 1, a scanner for radiating a laser beam is provided above the object building table (the work piece platform in Patent Document 1) on which an object is put. In order to enlarge the size of the object to be built, a scanner is generally fixed on a X-Y drive axis device such as a gantry (the cross-slide support in Patent Document 1) so as to be movable in the directions of X and Y axes.